This invention relates generally to step-and-repeat alignment and exposure systems utilizing a projection lens of the reduction type for the photometric printing of an image of a first object, such as a reticle, upon a second object, such as a semiconductive wafer, and, more specifically, to apparatus for use in such systems to facilitate alignment of the semiconductive wafer with respect to the reticle.
In U.S. patent applications Ser. Nos. 026,722 and 053,995 an improved step-and-repeat alignment and exposure system is disclosed including a main stage movable along orthogonal axes of motion of the system, another stage mounted above the main stage for holding a main reticle containing a level of microcircuitry, a substage mounted on the main stage and provided with a reference mark that may be employed to facilitate alignment of the main reticle or an image thereof with respect to the orthogonal axes of motion of the system, and a vacuum chuck mounted on the main stage for holding a semiconductive wafer (containing an array of different regions comprising inchoate semiconductive dice. The system enables the semiconductive wafer to be prealigned as a whole (i.e., globally aligned) with respect to the orthogonal axes of motion of the system and, hence, with respect to the main reticle or an image thereof, and then to be precisely aligned region by region with respect to the main reticle or an image thereof in order that the level of microcircuitry contained on the main reticle may be successively photometrically printed on the semiconductive wafer at each of those regions in alignment with other levels of microcircuitry previously or yet to be printed at each of those same regions. Thus, the system also includes a projection lens of the reduction type that may be employed to allow direct viewing and alignment of the reference mark with respect to the orthogonal axes of motion of the system during a reference mark set-up operation, of the semiconductive wafer with respect to the orthogonal axes of motion of the system during the global alignment operation, and of each region (when selected) of the semiconductive wafer with respect to the main reticle or an image thereof during the precision region-by-region alignment operation.
In addition, the system includes prealignment apparatus comprising a dual channel objective lens unit and a corresponding pair of prealignment reticles that may be employed to allow global alignment of the semiconductive wafer with respect to the orthogonal axes of motion of the system more conveniently than can be performed with the projection lens. The system further includes a fixed light source for illuminating the prealignment reticles and, hence, global alignment mark containing portions of the semiconductive wafer with nonexposure light during the global alignment operation (when performed with the prealignment apparatus), a controllable light source for selectively illuminating selected portions of the main reticle and, hence, of the semiconductive wafer with either exposure light of a wavelength for which the projection lens is corrected or nonexposure light of a different wavelength during the precision region-by-region alignment operation (and also during the global alignment operation, when performed with the projection lens), and a compensating lens that must be employed to compensate for the difference in wavelength of the exposure and nonexposure light when the latter is selected.
Although in this system the projection lens may normally be employed to allow direct viewing, global alignment, and precision region-by-region alignment of the semiconductive wafer, there are certain situations in which such direct viewing and alignment cannot be performed while employing the projection lens due to interference patterns that may be created in an image plane of the projection lens (for some photoresist coatings or other surface conditions of a semiconductive wafer being viewed in that image plane) at the wavelength of light for which the projection lens is corrected or compensated. There may also be situations in which such direct viewing and alignment may not be performed as conveniently while employing the projection lens, such as when nonexposure light is required for viewing and alignment. The controllable light source must then be switched to illuminate selected portions of the main reticle and, hence, of the semiconductive wafer with nonexposure light rather than exposure light, and the compensating lens must be switched into place to compensate for the difference in wavelength of the nonexposure light and the exposure light for which the projection lens is corrected.
Although the prealignment apparatus can be employed to perform global alignment of the semiconductive wafer with respect to the orthogonal axes of motion of the system in place of the projection lens, the prealignment apparatus cannot be employed to perform precision region-by-region alignment of the semiconductive wafer with respect to the main reticle or an image thereof. The reference mark, which is directly employed in the alignment of the main reticle or an image thereof with respect to the orthogonal axes of motion of the system, is located on the main stage at a position (adjacent to one side of the vacuum chuck) spaced away from the prealignment apparatus such that the reference mark cannot be directly employed with the dual channel objective lens unit of the prealignment apparatus to align both prealignment reticles or images thereof with respect to the orthogonal axes of motion of the system and, hence, with respect to the main reticle or an image thereof during a prealignment reticle set-up or checking operation. It is therefore necessary to employ the projection lens in aligning a set-up wafer with respect to the orthogonal axes of motion of the system and the channel objective lens unit of the prealignment apparatus in aligning the prealignment reticles or images thereof with respect to the set-up wafer in order to perform a prealignment reticle set-up or checking operation. The prealignment reticles cannot be set up with sufficient precision in this manner to be employed in the precision region-by-region alignment operation. Moreover, a pair of prealignment reticles or images thereof cannot be aligned relative both to one another and to the main reticle or an image thereof with sufficient precision to be employed in the precision region-by-region alignment operation. The foregoing factors also limit the accuracy with which the global alignment operation can be performed while employing the prealignment apparatus.
Since the reference mark cannot be directly employed with both the projection lens and the dual channel objective lens unit of the prealignment apparatus in the main reticle alignment and prealignment reticle set-up operations, the alignment of the main reticle and the prealignment reticles or images thereof with respect to the orthogonal axes of motion of the system and with respect to one another cannot be conveniently checked. Moreover, it is not readily possible to check the relative spacing between the main reticle and the prealignment reticles or images thereof from time to time as would be desired to permit slight adjustments in the step-and-repeat positioning of the main stage to compensate for changes in that relative spacing and concomitantly in the relative spacing of adjacent regions of the semiconductive wafer due to changes in environmental conditions such as temperature and the like (i.e., to permit reconciliation of the system with environmental changes affecting the step-and-repeat positioning of the main stage). Thus, more careful environmental control is required than would otherwise be necessary if such reconciliation of the system were possible.
When employing the prealignment apparatus to perform the global alignment operation, the global alignment mark containing regions of the semiconductive wafer are illuminated by nonexposure light passing through the prealignment reticles from the fixed light source. However, it may sometimes be difficult to locate the global alignment marks (on the semiconductive wafer) to be aligned with respect to the prealignment reticles or images thereof since the global alignment marks may be masked by the prealignment reticles themselves and since the illumination passing through each prealignment reticle does not illuminate the entire field of view of the corresponding objective lens of the dual channel objective lens unit of the prealignment apparatus.
It is the principal object of the present invention to provide this step-and-repeat alignment and exposure system with an auxiliary optical unit that may be employed with a relocated reference mark to allow direct viewing, global alignment, and precision region-by-region alignment of the semiconductive wafer, for example, when the projection lens cannot be so employed or may not be so employed as conveniently.
Another object of the present invention is to provide a single channel auxiliary optical unit that may be employed with a corresponding single auxiliary reticle and with the relocated reference mark to allow direct global alignment of the semiconductive wafer with greater precision and fewer parts than the original prealignment apparatus, and to allow direct precision region-by-region alignment of the semiconductive wafer not possible with the original prealignment apparatus.
Another object of the present invention is to relocate the reference mark so that it can be directly employed both with the projection lens to align the main reticle or an image thereof with respect to the orthogonal axes of motion of the system and with the single channel auxiliary optical unit to align the corresponding single auxiliary reticle or an image thereof with respect to the orthogonal axes of motion of the system.
Another object of the present invention is to relocate the reference mark as in the immediately preceding object so that the reference mark can be conveniently employed at any time with the projection lens and with the single channel auxiliary optical unit to allow checking and adjustment, if necessary, of the alignment of the main reticle and the single auxiliary reticle or images thereof with respect to the orthogonal axes of motion of the system and, hence, with respect to one another, and also to allow checking for changes in the relative spacing between the main reticle and the single auxiliary reticle or images thereof and thereby permit reconciliation of the system, if necessary, with environmental changes causing changes in that relative spacing and affecting the step-and-repeat positioning of the main stage.
Another object of the present invention is to provide the step-and-repeat alignment and exposure system with a frame of reference that may be employed, for example, in reconcilation of the system as in the immediately preceding object.
Still another object of the present invention is to frontally illuminate the entire field of view of the single channel auxiliary optical unit with white nonexposure light (i.e., white light exclusive of any exposure wavelengths) that does not pass through the single auxiliary reticle and therefore illuminates a larger portion of the semiconductive wafer, when the auxiliary optical unit is employed in viewing and aligning the semiconductive wafer, than does the original fixed light source employed with the prealignment reticles.
These and other objects of the present invention are accomplished according to the illustrated preferred embodiment of the present invention by replacing the original prealignment apparatus of this step-and-repeat alignment and exposure system with a single channel auxiliary optical unit and with a single auxiliary reticle positioned above and axially aligned with a main objective lens of the single channel auxiliary optical unit, and by relocating the substage and the reference mark provided thereon at a position on the main stage (adjacent to the opposite side of the vacuum chuck from their original position) where the reference mark can be directly employed with the projection lens and with the single channel auxiliary optical unit in aligning the main reticle and the single auxiliary reticle or images thereof with respect to the orthogonal axes of motion of the system and, hence, with respect to each other. The step-and-repeat alignment and exposure system is also provided with a detector unit for determining an initial reference or home position of the main stage and facilitating reconcilation of the system with respect to that home position. In addition, the fixed light source originally employed for illuminating the prealignment reticles is replaced by an auxiliary light source unit for illuminating the single auxiliary reticle with white nonexposure light and for selectively frontally illuminating the entire field of view of the single channel auxiliary optical unit with white nonexposure light that does not pass through the single auxiliary reticle.